Spiral-webbed mandrel

ABSTRACT

An mandrel for extrusion of hollow bodies, such as, polyvinyl chloride (PVC) hard, compact, and foamed pipes, includes an inner collar and an outer collar, and webs radially joining the inner collar and outer collar. The webs can be arranged twisted and/or offset in radial direction.

RELATED APPLICATION

[0001] This application claims priority under 35 U.S.C. § 119 to GermanApplication No. DE 102 05 842.3, filed Feb. 13, 2002, the entireteachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Prior art extrusion methods have used double-webbed mandrel diesfor extrusion of hollow bodies, such as polyvinyl chloride (PVC) hard,compact, and foamed pipes. The double-webbed mandrel die has an innercollar, an outer collar, and a series of webs, which radially join theinner collar and the outer collar.

[0003] Double-webbed mandrel dies for production of pipe are known, forexample, from a brochure of Battenfeld Extrusionstechnik GmbH entitled“Pipe Extrusion Systems” dated October 2000. Double-webbed mandrel diesare also known in U.S. Pat. No. 4,509,907, issued to Ratheiser on Apr.9, 1985, the entire teachings of which are incorporated herein byreference.

[0004] Prior art dies shape the outer and the inner contour of the pipeusing a sleeve and a mandrel. The sleeve and the mandrel are be joinedtogether to withstand the wall friction forces and the resultingpressure forces of the extrudate (melt). The mandrel is joined to thesleeve by a series of webs. However, the pressure buildup in the melt isso great that adequate pipe quality is only achieved with sufficientsubsequent compression and a limiting of the web height.

[0005] In known double-webbed pipe mandrel dies, the connection betweenthe sleeve and the mandrel is provided by two sets of series of webs,which are staggered in a circumferential direction and joined by amiddle collar. This technique offers two advantages. First, the pressurebuildup in the melt is not as severe because less compression work isneeded and a larger annular gap between sleeve and mandrel can becreated. Furthermore, this allows for excellent control of wallthickness of the pipe when using a high production rate. There are nocontinuous web markings and the mechanical properties of the pipe areincreased when using this type of double-webbed pipe mandrel.

SUMMARY OF THE INVENTION

[0006] To control and regulate the extrusion process, supply lines arelead through the double-webbed mandrel head. The supply lines may bepower lines, or control lines used in conjunction with temperatureprobes, heating elements or the like devices used for interior controlof the extrusion process. One drawback in the prior art techniques isthat the supply lines need to be heavily folded because the webs arestaggered between the individual collars. Thus, it is necessary to spanthe offset between the two webs within the middle collar by the supplyline. In the known systems, a groove is provided in the middle collar tolead the supply lines through the mandrel head. Once the supply linesare in place, the groove is closed to protect the supply lines. Themanufacturing and assembly expense required for this known technique isconsiderable, and is a large portion of the entire manufacturing expensefor the die. Furthermore, the operating safety of the die isconsiderably reduced because of the above expenditures (malfunctioningof the improperly laid supply lines).

[0007] In accordance with one embodiment of the invention, a method andapparatus are provided which can eliminate the need for the middlecollar, such that the supply lines do not have to be folded. Thespiral-webbed mandrel die includes an inner collar, an outer collar, andwebs radially joining the inner collar and the outer collar. In aparticular embodiment, the webs are twisted and/or offset in a radialdirection.

[0008] As a further enhancement, the axis of rotation of the web is inthe vicinity of the onflow curve, so that at least no continuous webmarkings are formed. The flow of melt arrives at the webs in a radialline, but converges at a meeting point that is radially offset from theonflow axis.

[0009] This web arrangement allows for a borehole to extend from theouter collar through at least one web to the inner collar. Using theborehole, the supply lines, for example, power lines, can be led fromthe outside of the outer collar through the web to the inside of theinner collar, without accounting for the offset of the webs.Consequently, the supply lines do not have to be folded.

[0010] The axis of rotation of the webs near the onflow curve canproduce a relatively large displacement of the outflow axes in thecircumferential direction of the die. Another feature of the webgeometry is that it can be streamlined for a particular torsion. Thatis, the cross-sectional geometry can vary along the axis of rotation andin keeping with technical flow requirements to achieve increasedmechanical properties of the pipe.

[0011] The die geometry can allow for continuous shifting of the outflowaxes in the circumferential direction, which produces a blurring of theweb markings in the extruded pipe and provide for increased mechanicalproperties of the pipe. The supply borehole can be made concentricallyto the axis of rotation by choosing the die geometry. This reduces thefabrication and assembly cost and the consequent operating security hasbeen distinctly enhanced. The middle collar can now be eliminated.

[0012] Another feature of the web arrangement is the torsion about theaxis of rotation or the offset from the radial axis, the positioning ofthe axis of rotation, and the streamlined conditions in each segment ofthe web. In particular embodiments, the trend of the angle of rotationis varied along the axis of rotation, that is the offset between theonflow curve and outflow curve is different.

[0013] More advantageously, the cross-section of one web is streamlined,for example, teardrop-shaped, thereby advantageously influencing theconditions of flow around the web.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of various embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0015]FIG. 1 is a perspective view of a spiral-webbed mandrel inaccordance with one embodiment of the invention;

[0016]FIG. 2 is a perspective view of a spiral-webbed mandrel inaccordance with an alternative embodiment of the invention;

[0017]FIG. 3 is a perspective view of a spiral-webbed mandrel inaccordance with another embodiment of the invention; and

[0018]FIG. 4 is a perspective view of a spiral-webbed mandrel inaccordance with another embodiment of the invention;

DETAILED DESCRIPTION OF THE INVENTION

[0019] As shown in FIG. 1, a mandrel die is depicted, in which an innercollar 1 and an outer collar 2 are joined by webs 3. A borehole 4extends from the outside of the outer collar 2 via one of the webs 3into the inner collar 1 so that supply lines can be led into the devicefrom the outside. The supply lines may be power lines, or control linesused in conjunction with temperature probes, heating elements or thelike devices used for interior control of the extrusion process.

[0020] These webs 3 are twisted about a radial axis relative to borehole4, so that the onflow curve 5 is radially offset from the outflow curve6. Embodiments of the invention place few limits on the configuration ofthe webs 3. The twisting or winding of a web, as shown in FIG. 2, can bespiral-shaped with a continuous onflow curve 5 and outflow curve 6 or,as shown in FIG. 3, discontinuous with a folded trend for the onflowcurve 5 and outflow curves 6. In all embodiments of the presentinvention, the melt is divided on the onflow side of the web, in theextrusion direction, and converges at the outflow side of the web so noweb markings occur to provide for increased mechanical properties of thepipe. The onflow curve 5, i.e., the region of a web which divides themelt, has a different trend seen spatially than the outflow curve 6,i.e., the region of the web behind which the melt again flows together,and a borehole 4 or a channel can be made through the web, in order tolead in the supply lines from the outside without excessively strainingthem.

[0021] As shown in FIG. 4, one alternative configuration of thespiral-webbed mandrel die is to offset the webs 3 in the radialdirection. The connection of a web 3 to the outer collar 2 has adifferent radial axis than the connection of this web to the innercollar 1. However, this configuration has limits, where the size of theoffset can only be chosen so a borehole 4 can still be brought in fromthe outside of the outer collar 2 into the inside of the inner collar 1.

[0022] With the proposed device, plastic parts having a hollow can beextruded, which applies not only to pipes, but also profile sectionssuch as window frames or the like.

[0023] While this invention has been particularly shown and describedwith references to various embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An apparatus for the extrusion of hollow bodiescomprising: an inner collar; an outer collar; webs radially joining theinner collar and the outer collar, that an outflow axis of each webbeing non-radial.
 2. The apparatus of claim 1, wherein the webs aretwisted in a radial direction.
 3. The apparatus of claim 1, wherein thewebs are offset in a radial direction.
 4. The apparatus of claim 1,wherein an axis of rotation of the web is in the vicinity of an onflowcurve.
 5. The apparatus of claim 1, wherein the webs are arranged sothat an outflow curve is offset radially from the onflow curve.
 6. Theapparatus of claim 1, wherein the webs are streamlined in cross-section.7. The apparatus of claim 6, wherein the webs are teardrop-shaped incross-section.
 8. The apparatus of claim 1, further comprising aborehole that extends from the outer collar through at least one of thewebs to the inner collar.
 9. The apparatus of claim 8, wherein theborehole is suitable to accommodate supply lines.
 10. The apparatus ofclaim 9, wherein the supply lines include power lines.
 11. The apparatusof claim 1, wherein the apparatus is configured to form hollow bodiesthat include PVC-hard, compact, and foamed pipes.
 12. The device ofclaim 1, wherein the webs have a continuous onflow curve.
 13. The deviceof claim 1, wherein the webs have a discontinuous onflow curve with afolded trend.
 14. A method for extrusion of hollow bodies, comprising:injecting a melt into a webbed mandrel; and in the webbed mandrel,dividing the melt around twisted webs which are offset in a radialdirection from an inner collar and an outer collar of the webbedmandrel.
 15. The method of claim 14, wherein an axis of rotation of theweb is in the vicinity of an onflow curve.
 16. The method of claim 15,wherein the webs are arranged so that an outflow curve is offsetradially from the onflow curve.
 17. The method of claim 14, wherein thewebs are streamlined in cross-section.
 18. The method of claim 17,wherein the webs are teardrop-shaped in cross-section.
 19. The method ofclaim 14, further comprising the step of boring a borehole that extendsfrom the outer collar through at least one of the webs to the innercollar.
 20. The method of claim 14, wherein the borehole is suitable toaccommodate supply lines.
 21. The method of claim 20, wherein the supplylines include power lines.
 22. An apparatus for extrusion of hollowbodies comprising: means for directing a melt around a series webs; andmeans for providing a linear borehole through at least one web in anaxial direction.